The present invention relates to novel mixtures which can be used as photoinitiators and are based on aromatic-aliphatic ketones, to their use as photoinitiators, in particular for the photopolymerization of ethylenically unsaturated compounds and for the ultraviolet hardening of printing inks, and to photopolymerizable systems containing such mixtures.
Photochemically initiated polymerization reactions have gained great importance in industry, in particular when rapid hardening of thin coatings is required, such as, for example, in the hardening of lacquer coatings and resin coatings on paper, metal and plastic or in the drying of printing inks. Compared with conventional methods for the printing and coating of articles, these processes are distinguished by a saving in raw materials and energy and by reduced pollution of the environment. Since, in these reactions, none of the reactants is as a rule capable of absorbing the photochemically active radiation to an adequate extent, so-called photoinitiators must be added.
These are capable of absorbing incident visible or ultraviolet radiation and forming active starter radicals when being excited. According to the kind of reaction involved after their photoexcitation, two groups can be differentiated. After being excited, a category of compounds directly decays into active starter radicals. These, in turn, initiate the polymerization. Another category of compounds, after being excited, first interacts with a second compound to produce the active starter radicals. These, in turn, initiate the polymerization. Herein, both categories of compounds are comprised by the term "initiators." The essential criteria for the selection of such initiators are, inter alia, the type of the reaction to be carried out, the relationship of the absorption spectrum of the initiator to the spectral energy distribution of the available radiation source, the solubility of the initiator in the reaction mixture, the storage stability in the dark of the reaction system to which the initiator has been added, and the extent to which the end products are influenced by residues of initiator which have remained therein, and/or of the products formed from the initiator during the photochemical reaction. In particular, the rate of reaction strongly depends on the initiator used. For this reason, there has been no lack of attempts to provide new initiators and new initiator mixtures showing increased reactivity in initiating the photopolymerization of ethylenically unsaturated compounds or the hardening of photopolymerizable systems.
Heretofore, initiators for the photopolymerization of unsaturated compounds have primarily been benzophenone derivatives, benzoin ethers, benzil monoketals, dibenzosuberone derivatives, anthraquinones, xanthones, thioxanthones and .alpha.-halogenoacetophenone derivatives. The industrial applicability of these substances, is, however, markedly restricted by a number of disadvantages. These include, inter alia, the low storage stability in the dark of many photopolymerizable systems to which these initiators have been added, a low chemical stability and an unduly low reactivity in initiating the photopolymerization of ethylenically unsaturated compounds or the hardening of photopolymerizable systems. Many of these known initiators cause yellowing of the polymers prepared with them. This yellowing is extremely undesirable in the coating of materials with a light background and in ultraviolet-hardened printing inks. For this latter field of application, the frequently low solubility of the known initiators in the photopolymerizable system is also a great disadvantage. Printing inks, as a rule, contain considerable quantities of colored pigments which absorb a large part of the energy radiated in. However, this part of the absorbed energy does not become photochemically effective. Thus, a relatively large quantity of initiator must be added in this case. The known initiators then frequently partially crystallize out. Apart from the fact that fractions which have crystallized out can no longer become effective for initiation after some time, the crystallites formed also damage the printing plates which consist of relatively soft materials.
Moreover, German Offenlegungsschrift No. 2,722,264 describes a number of hydroxyalkylphenones as photoinitiators. These display the disadvantages of the hitherto known initiators to a substantially smaller extent. These compounds, some of which are liquid, are markedly more soluble in the customary photopolymerizable systems than are most of the hitherto known initiators, which as a rule are solid. They also have good stability on storage in the dark and good chemical stability. In particular, however, when these initiators are used in photopolymerization, yellowing of the polymers is observed only to a substantially smaller extent than for the hitherto usual initiators. Notwithstanding the increased photoinitiating activity of these hydroalkylphenones as compared with the other known initiators, even these still have an inadequate reactivity in initiating the photopolymerization. As a result, relatively long hardening periods are still required, and this leads to a non-optimum utilization of the ultraviolet irradiation units.